A cellular wireless access telecommunications network (system) typically includes multiple base stations, also known as, for example, base transceiver station (BTS) in GSM, NodeB in WCDMA (UMTS), and evolved NodeB or eNB in LTE. A base station includes at least transmitting and receiving equipment to support wireless communication with a (possibly mobile) terminal, in standardization more formally known as UE (User Equipment). The range that can be covered with the transmitter/receiver in a base station is limited. The area that can be served by the transmitter/receiver of a base station is referred to as its “coverage area” or as the “cell.” As used herein, the term “cell” may refer to both the base station itself and to its associated coverage area.
A cell (base station) in a cellular network is typically connected to the remainder of the network via one or more backhaul links, for example, via optical fiber, via copper wire or wirelessly. A base station further includes processing capabilities, for example for the wireless transmission and reception and for handling the protocols specified between the base station and the terminal and between the base station and the network, including other cells.
In a cellular network, different cells may have different sizes, indicated e.g. as macrocells, microcells, picocells or femtocells in decreasing order of cell size. Cells may show a partial overlap with nearby cells or a smaller cell (e.g. picocell) may be entirely overlapped by a larger cell (e.g. macrocell). Multiple cells may thus form a cellular network providing near contiguous coverage in a very large area.
In a cellular network it is common that each cell (base station), when in operation, transmits broadcast signals. Such signals are known as, for example, BCH (Broadcast CHannel) in GSM, as CPICH (Common Pilot CHannel) in WCDMA (UMTS) and as RSs (Reference Signals) in LTE. The same or separate broadcast signals are used to indicate a cell's (base station's) presence and to broadcast information about the cell (system information), for example, the cell identity and information about the configuration of the cell and/or about the cell's resources, such as e.g. which channel to use in order to initiate contact with the cell. Such broadcast signals allow terminals to make measurements on the broadcast signals, e.g. to determine the strength of the signal received by the terminal, and to receive the cell's system information. The broadcast signals are usually transmitted as long as the cell is in operation. The transmit power involved in broadcasting these signals may consume up to 20% of the cell's maximum transmit power, also when the cell does not actually exchange data with a terminal in the cell or when there is no terminal at all in the cell.
In a cellular wireless network it is common to distinguish a terminal to be in an ‘idle mode’ or in an ‘active mode’. In the active mode, the terminal is able to exchange data (e.g. sending/receiving an e-mail or making a phone call) via a cell in which the terminal is located. This requires resources in the network (e.g. frequencies and/or codes) and also requires the terminal and the network to provide power for the purpose. In the idle mode the terminal is not able to exchange data and, therefore, does not require the above resources and consumes less power. A terminal in the idle mode only regularly listens to signals broadcast by the cells and selects a ‘best cell’, for example the cell with the signal that the terminal receives as strongest. A terminal in the idle mode also monitors the paging channel transmitted by the selected cell for a paging message addressing the terminal. Such an (idle mode) terminal is said to ‘camp on’ the selected cell. When, for example because of terminal mobility, a different cell is identified as best cell, the terminal may re-select the different cell as ‘best cell’ and camp on the newly selected cell. It should be noted that a terminal in the idle mode normally does not inform the cell and/or the network about which cell the terminal is camping on, also not when re-selecting a different cell as best cell. When the terminal re-selects to a cell which is found to be in a different location area (LA or RA—routing area), which the terminal may determine from the cell's system information, then the terminal initiates contact with the network via the newly selected cell to perform an LA or RA update procedure, and subsequently returns to the idle mode. Thus, the network is made aware of the LA/RA the idle terminal is located in. A LA/RA commonly comprises multiple cells, as configured by the network operator. Consequently, the network is not aware on which cell an idle mode terminal is camping on, it is only aware in which LA/RA an idle terminal is (expected to be) located.
In a cellular wireless access telecommunications network a terminal and the network need to set up a session when the terminal requests a service or is being paged. This involves a terminal in the idle mode making a transition to the active mode. In an LTE network, for example, a session setup is a two-step process. If service is initiated by the network, the network performs a paging procedure, where a paging message is broadcast in all cells where the network expects the terminal to be camping on (RA/LA). When the terminal receives a paging message addressing the terminal, or if service is initiated by the terminal without having been paged, in a first step the terminal performs a random access channel (RACH) procedure towards the cell it currently is camping on to establish a Radio Resource Control (RRC) connection. When successful, in the second step, the RRC connection with that cell is used to negotiate resources for and to establish a data connection between the terminal and that cell. Then the wireless exchange of user data between the terminal and the cell is possible.
For the LTE network as well as for other legacy networks such as e.g. GSM and UMTS, all transmissions (be it signaling or data) typically occur between the terminal and a single cell which is the same cell that the terminal was camping on when it was in idle mode.
Recently, a new, more energy efficient, network architecture is being developed. One aspect in the new architecture is the use of relatively small cells. High bit rate data connections can be much more efficiently provided with a larger number of (at least partially overlapping) small cells (e.g. microcells, picocells, femtocells) than with a fewer number of larger cells (e.g. macrocells). A further aspect in the new architecture is that the power consumption of a cell is envisioned to scale, as much as possible, with the service actually provided (e.g. with the number of active terminals served, with the bit rate provided to a terminal, with the distance covered by the connection to a terminal, etc.). One approach for realizing this vision includes putting those cells that do not actually serve an active terminal into a power-save mode, e.g. switching those cells almost completely off. Another, complementary, approach includes significantly reducing or refraining from transmitting broadcast signals that are common in conventional networks. The transmission of these broadcast signals causes a large overhead, in particular for cells operating at less than full load.
The new architecture envisions distinguishing between different types of cells. A first type of cells, in this text referred to as ‘SA-cell’ is primarily optimized to support the wireless exchange of data with active terminals. The energy-efficiency improvements as outlined above are focused on the SA-cells. A second type of cells, in this text referred to as ‘LA-cell’ is primarily optimized for other functions in a cellular network, including those also found in conventional networks. Thus, it is envisioned to reduce the overhead in the system to that attributed to the LA-cells.
A LA-cell typically covers a larger area, for example comparable to that of a conventional macrocell. The LA-cells together provide near contiguous coverage in the area desired to be covered, much like in a conventional network. A LA-cell may transmit broadcast and system information, much like a conventional cell; an idle terminal may camp on a LA-cell and may also initiate a signaling connection with the LA-cell, e.g. to perform an LA/RA update or to detach from the network.
A SA-cell covers a smaller area, for example comparable to that of a conventional microcell, picocell or femtocell. The SA-cells together may support a certain bit rate in the near-contiguous area desired to be covered. An SA-cell only transmits signals when and in so far it is needed; it may be regarded to be normally ‘off’ or in a power-save or stand-by mode. An idle terminal also does not camp on a SA-cell. Although such a network has been referred to as a “Beyond Cellular Green Generation” (BCG2) network, this term may change in the future. Therefore, in the context of the present application, a network having such architecture will be referred to as an “energy-efficient cellular wireless network.”
In an energy-efficient cellular network, if session setup is network-initiated, this is preceded by the terminal receiving a paging message via the LA-cell it is camping on. The session setup in an energy-efficient cellular network may be sub-divided into two parts. The first part includes the establishment of a signaling connection between the terminal and the LA-cell it currently is camping on, which may include a RACH procedure and RRC connection set-up, similar to legacy networks. After a signaling connection between the terminal and the LA-cell has been established, the second part includes the establishment of a data connection (data session) with an appropriate SA-cell.
Note that, in an energy-efficient network according to this architecture, it may sometimes not be possible to identify an appropriate SA-cell immediately. This may happen e.g. because all SA-cells in the vicinity of the terminal may be in a power-save mode and do not transmit a suitable signal. It may also be the case that some SA-cell is active and that the terminal detects a suitable signal from the SA-cell, but that the active SA-cell cannot optimally support the requested data session from an energy saving perspective (e.g. there is an inactive SA-cell in a better position, e.g. much closer to the terminal). It may also be the case that some SA-cell is active but that the active SA-cell cannot optimally support the requested data session from a quality of service (QoS) perspective (e.g. the active SA-cell cannot support the data session with the requested bit rate, while other inactive SA-cell(s) can).
Setting up a session in a network according to this new architecture is different from that in legacy networks. One difference is that the terminal issues a request message to a ‘best LA-cell’ an idle terminal is camping on but that this cell is, normally, not going to serve the terminal (i.e., the data connection for exchanging the actual user data is set up with another SA-cell). Another difference is that the SA-cell's RAT (Radio Access Technology) to support the data connection need not be the same as the LA-cell's RAT to support the terminal in idle mode, which allows optimization of one or both RATs separately for their respective primary purposes. Yet another difference is that, in the new architecture, the ‘best SA-cell’ to support the data connection still needs to be found. Consequently, as a part of the session setup procedure in an energy-efficient network, an appropriate cell (SA-cell) needs to be selected to support the data connection with the terminal. To ensure high quality and/or user experience, the session setup, which includes both identifying an appropriate cell and establishing a data connection with it, is preferably performed quickly.
In existing mobile radio networks, such as LTE, UMTS and GSM networks the terminal performs a random access procedure as initial step of the session set-up. The random access procedure is by default contention-based, where the UE randomly selects from a set of predetermined random access preambles and transmits the selected preamble in a random access slot (in time and/or frequency domain) of the target cell. It should be noted, however, that for LTE there is also a contention-free RACH procedure that can be used in the following time-critical procedures:
a) The terminal performs a handover between the source and the target cell. In this situation it uses a contention free (i.e. a unique) preamble for the uplink random access transmission at the target cell. This is needed to avoid collisions (i.e. more than one terminal is using the same preamble simultaneously) and speed-up the random access as the terminal is in the process of handing over from the source to the target cell.
b) The terminal has to perform uplink (UL) transmission (e.g. sending an ACK) on the uplink for cases when downlink (DL) data transfer is resumed and the uplink synchronization is lost. In order to restore the UL synchronization the terminal performs again collision-free RACH procedure (i.e. it uses a unique random access preamble) in order to speed-up the UL resynchronization as there is limited time to receive the uplink ACK for the transmitted DL data.
The LTE contention-free RACH transmission via the unique (contention-free) access preamble is implemented such that the terminal selects from the set of contention-free preambles broadcast in the system information.
In LTE there are fixed number of 64 RACH preamble signatures (in UMTS this number is not fixed) based on the Zadoff-Chu (ZC) sequence with a particular length. In UMTS the RACH preambles are based on Pseudo Noise (PN) sequences. The ZC sequences enable good detection at cell edge (i.e. low SINR conditions), reduced intra-cell interference from colliding preambles, and acceptable eNodeB receiver complexity.
The 64 RACH preambles are broadcast by the cell as part of system information including indication which portion of the preambles is reserved for contention-free RACH procedure. At mobile or network originated call the terminal randomly selects a RACH preamble (excluding those reserved for contention-free RACH) and a RACH time slot and sends it to the eNodeB. It is possible that more than one UE transmit the same preamble in the same random access slot, leading to a contention-based RACH procedure. Ideally, if there is only one terminal sending its RACH preamble in the particular RACH time slot, then the network can decode it and respond with a Random Access Response (RAR) including the timing advance, Cell Radio Network Temporary Identifier (C-RNTI) that might be definite if there is no collision, and the uplink grant for the subsequent data transmission for the terminal. Note that there is a limited time interval where the terminal expects the RAR, so if the preamble is not detected in case of collisions or bad radio conditions or too low uplink transmit power the terminal retransmits the preamble again. The terminal follows the UL grant and sends a L2/L3 message (e.g. RRC connection set-up) as instructed and includes its own unique identity. Note that in case of collision two (or more) terminals will send its L2/L3 messages on the same uplink resources with their respective identities. At the network side the eNodeB sends an early contention message, addressed to the C-RNTI and echoing the terminal identity decoded in the L2/L3 message received in the previous step. This early contention resolution message supports hybrid automatic repeat requests (HARQ), i.e. it requires ACK feedback from the UE. This ACK feedback is given only by the terminal that has correctly decoded the early contention message and recognized its own identity. Other terminals not recognizing their respective identities understand that there was a collision, transmit no HARQ feedback, quickly exit the current RACH procedure and start another one. Therefore, upon reception of the early contention resolution message the terminal has the following options: (a) the message is correctly received, own identity is detected and ACK is sent (i.e. RACH procedure is finished); (b) the message is correctly received, the identity is not recognized (i.e. contention resolution) and no feedback is sent (e.g. discontinuous transmission DTX); (c) the terminal fails to decode the message or misses the DL grant, no feedback is sent (DTX).
In some time-critical cases (e.g. handover or sending uplink ACK when DL data transfer is resumed), the eNodeB has the option to arrange contention-free access for the UE by assigning the UE a dedicated preamble via a dedicated signaling. Theoretically, it is still possible to have time collision i.e. the active terminal uses the same PRACH resource as with terminal transiting from idle to active. Note that two or more active terminals using the same PRACH resources is not expected as that would mean issuing handover (HO) command or requesting ACK of DL data at exactly the same time instant for two or more mobiles. In this case due to the different preambles and the properties of the Zadoff-Chu (ZC) preamble sequences used, it is still possible to distinct among the different terminals. Contention-free access is generally faster than contention-based access, since it does not require subsequent contention resolution process. It ends when the eNodeB sends the RAR message.
In UMTS, the available PRACH random access slots (totally 14 slots per 20 ms) are grouped into so called RACH sub-channels. Each of the RACH sub-channels is allocated to one of 8 Access Service Classes (ACSs), which denote the priority of the service for which RACH request is being made. The set of available preambles and the set of available RACH sub-channels for each ACS are provided via system information. At random access, the UE randomly select one preamble and one random access slot from the sets corresponding to the given ACS of the service. The UE also has to estimate initial power with which it is to send the selected preamble. After sending the preamble, the UE will listen to the downlink Acquisition Indicator CHannel (AICH) for the acknowledgement of the network. If a positive acknowledgement is detected, the UE transmits the random access message over the PRACH. If a negative acknowledgement is detected, the UE exits the random access procedure. If no positive or negative acknowledgement is detected, due to either collision or too-low uplink transmit power, the UE chooses the next available random access slot and randomly selects a new preamble from the sets corresponding to the given ACS. The UE also increases the transmit power by a predetermined step, and transmit the newly-selected preamble during the newly chosen random access slot.
US patent application US 2008/0261570 discloses the possibility to signal SIB7 information of UMTS in each paging message in order to provide the terminal with the random access parameters in the case of network-originated calls. The SIB7 information includes some random access parameters: Uplink Interference, Dynamic Persistence Level per Physical Random Access Channel and Expiration Time Factor. The purpose is that, in case that the random access parameters the terminal received and stored before are not valid anymore (decided by the parameter Expiration Time Factor), and therefore the terminal does not need to wait for the next transmission of the SIB7 information (typically with transmission period of 80 ms), and thus extra random access delay is avoided. No information of random access slot and preamble is signaled in the paging message.
Patent application WO 2013/037875 discloses, amongst others, user terminals transmitting an information request message (IRM) towards the plurality of the SA-cells in its surroundings. The SA-cells are connected with a (fixed or wireless) backhaul link with the overlaying LA-cell. The IRM based session set-up uses the IRM signal strength measurements at the surrounding SA-cells that are triggered and instructed by the LA-cell to receive the IRM message. The signal strength measurements at the SA-cell receiving the IRM message are reported back to the mobile terminal so the terminal can select the SA-cell or the IRM signal measurements are forwarded via the backhaul link from the receiving SA-cells towards a decision unit in the network that decides which SA-cell should serve the mobile terminal.
It is an object of the present invention to improve the random access procedure for establishment of a data connection between a terminal and a SA-cell in an energy-efficient network, such as e.g. a BCG2 network.